The present invention relates to a technology for controlling the shifts of an automatic transmission.
A vehicular automatic transmission generally uses a plurality of planetary gear sets, each of which is composed of revolving components such as a sun gear, a ring gear and a carrier. The automatic transmission is constructed by connecting predetermined ones of the revolving components, by connecting any of the revolving component selectively to an input shaft through clutch means, by fixing another revolving component selectively by brake means, and by connecting an output shaft to still another revolving component. In the automatic transmission thus constructed, the number of speed stages to be set, the gear ratios at the individual speed stages, and the number of revolution, loads or torques of the revolving components are varied in dependence upon the number of the planetary gear sets used, the manner how to connect the individual revolving components, and the number of and the manner how to dispose frictional engagement means such as the clutch means or the brake means, so that a remarkable variety of automatic transmissions can be constructed. All the structures could not be put into practical use, although they could be made in principle. It is not easy in the least to create a structure which can sufficiently meet the practical requirements such as feasible production, small size and low weight, ample shift controllability and excellent durability. For example, even an automatic transmission using three planetary gear sets to set forward five and reverse one speed stages might have an excessively large gear ratio for the reverse stage in dependence upon the manners how to arrange the frictional engagement means such as the clutches and how to adopt the gear ratio (i.e., the ratio of tooth numbers of the sun gears and the ring gears). For this automatic transmission, it would be necessary to increase the capacities of the frictional engagement means, thus raising a problem that the durabilities of the frictional engagement means and the bearing might be deteriorated. In order to solve this problem, as disclosed in Japanese Patent Laid-Open No. 60-57036, there is proposed an automatic transmission which is constructed to have a reverse gear ratio approximate to that for the forward 1st speed.
The structure of the automatic transmission according to this proposal will be briefly described in the following. This automatic transmission is composed mainly of three single pinion type planetary gear sets and is constructed by connecting the sun gear of the first planetary gear set and the sun gear of the second planetary gear set always or selectively, by connecting the sun gear of the first planetary gear set and the carrier of the second planetary gear set, if necessary, through a clutch, by connecting the ring gear of the second planetary gear set and the ring gear of the third planetary gear set, by connecting the carrier of the first planetary gear set with those ring gears, and by connecting the carrier of the second planetary gear set and the sun gear of the third planetary gear set through a clutch. The input shaft is connected through a clutch to the sun gears of the first and second planetary gear sets, which are connected to each other, and further to the ring gear of the first planetary gear set through another clutch. On the other hand, the output shaft is connected to the carrier of the third planetary gear set. As the brake means for stopping the revolutions, moreover, there are provided a brake for fixing the sun gear of the first planetary gear set and the sun gear of the second planetary gear set and a brake for fixing the sun gear of the third planetary gear set. In the automatic transmission thus disclosed in Japanese Patent Laid-Open No. 60-57036, for the forward 1st speed, the revolutions of the carrier of the second planetary gear set are blocked by engaging both the brake for fixing the sun gear of the third planetary gear set and the clutch for connecting the carrier of the second planetary gear set to that sun gear. For the reverse stage, too, the carrier of the second planetary gear set is fixed. And, the gear ratios at these forward 1st speed and reverse stage are given approximate values.
Incidentally, the structure, in which a one-way clutch is interposed between the carrier of the second planetary gear set and the casing, is shown in skeleton diagram in the aforementioned Japanese Patent Laid-Open No. 60-57036. However, this Laid-Open has failed to disclose the corresponding clutch and brake application chart and describe the speed stages to be set. Therefore, it is conceivable that the structure arranged with the additional one-way clutch sets the individual speed stages like other embodiments disclosed.
The automatic transmission thus disclosed in the prior art is enabled to set forward seven speeds and reverse one speed as a whole by adding 2.5th and 3.5th speeds to the main speed stages of forward five and reverse one speeds. However, the combination of the engagement/release of the clutches and the brakes for setting each of the speed stages is only one. In the automatic transmission of the prior art, therefore, the shift shocks may be deteriorated, or a complex control may be inevitably required for preventing the deterioration of the shift shocks. Specifically, one requisite to be enumerated for improving the shift controllability is that the number of the frictional engagement means to have their engagement/release states switched for a shift must be as small as possible. In the aforementioned automatic transmission, however, for the forward 3rd speed, the following three frictional engagement means are engaged: the clutch (as will be tentatively called the "first clutch") for connecting the input shaft and the sun gear of the first planetary gear set; the clutch (as will be tentatively called the "second clutch") for connecting the input shaft and the ring gear of the first planetary gear set; and the brake (which will be tentatively called the "second brake") for fixing the sun gear of the third planetary gear set. For the forward 4th speed, on the contrary, the following totally four clutches are engaged in addition to the aforementioned first and second clutches: the clutch (as will be tentatively called the "fourth clutch") for connecting the sun gear of the first planetary gear set and the carrier of the second planetary gear set; and the clutch (as will be tentatively called the "fifth clutch") for connecting the carrier of the second planetary gear set and the sun gear of the third planetary gear set. As a result, the engagement/release states of the three engagement means of the fourth and fifth clutches and the second brake have to be switched in case of a shift between the forward 3rd and 4th speeds. This makes it difficult to hold the engagement/release timings of those frictional engagement means proper at all times. This causes problems that the shift shocks are enlarged if the engagement/release goes out of timing and that the oil pressure to be fed to or drained from the hydraulic servomechanism has to be finely controlled to make the engagement/release timing proper.
Incidentally, it is well known in the art that the shift control of the automatic transmission is performed by using as its parameters both the engine load represented by the throttle opening and the vehicle speed. In case, therefore, the shift lever is moved to the drive range while the vehicle is in its stop position, the speed stage is set at first to the 1st speed. Because this 1st step has the largest gear ratio in the forward stages, the high torque may be abruptly applied to the output shaft to cause a phenomenon that the vehicular body has its rear diving. In order to lighten this so-called "squat", the following is the current practice of the prior art. In case the 1st speed is selected in the state of satisfying the predetermined conditions that the vehicle speed is not higher than a predetermined reference value and that the cooling water temperature is not higher than a predetermined level, the controls are made by setting the speed stage temporarily to another speed stage, e.g., the 3rd speed having a larger gear ratio and then to the 1st speed. Let the case be considered, in which the squat reduction control is to be performed in the automatic transmission disclosed in the aforementioned Japanese Patent Laid-Open No. 60-57036. In this case, the second clutch, the third clutch, the fifth clutch and the second brake are engaged for the 1st speed, whereas the first clutch, the second clutch and the second brake are engaged for the 3rd speed. If the speed stage is to be set temporarily to the 3rd speed and then to the 1st speed, it is necessary to release the first clutch and to engage the third clutch and the fifth clutch. This increases the number of the frictional engagement means to have their engagement/release states switched. As a result, the engagement/release timings are difficult to hold properly, and the shift shocks might be increased even if the squat could be reduced. It is, therefore, conceivable to reduce the squat by setting the speed stage temporarily to the 4th speed in place of the 3rd speed. For a subsequent shift to the 1st speed of this case, the first and fourth clutches are released, and the third clutch is engaged. In this case, too, the number of the frictional engagement means having their engagement/release states switched is increased to raise the possibility of the shift shocks. After all, in order to reduce the squat of the aforementioned automatic transmission of the prior art, the 2.5th speed has to be passed, in case of a shift to the 1st speed from a speed stage set temporarily prior to the 1st speed stage, so that the number of frictional engagement means having their engagement/release states switched may be reduced to two or less. Since, however, the 2.5th speed is accompanied by a high output shaft torque, it is difficult to perform a sufficient squat reduction control.